Binary beam system

ABSTRACT

A digital direction finder system for radio waves in which the individual bits of the code indicating direction are obtained by comparing and successively partially resolving errors in the digitized phase angle displacement indicated by plural pairs of antennas having progressively different spacing between the respective pairs. The spacing between the closest pair of antennas is more than one-half of the wavelength so that larger and higher gain antennas may be employed. The system may be operated over a wide range of frequencies where the highest frequency may be many times greater than the lowest frequency.

finite tates atent 1191 1111 3,852,754 Worrell Dec. 3, 19741 1 BINARYBEAM SYSTEM Primary Examiner-Maynard R. Wilbur Assistant E.raminer-T. M.Blum 1 [75] mentor Edsel A Bethesda Md Attorney, Agent, or Firm-A1fredB. Levmc; Alan C. [73] Assignee: Litton Systems, 1nc., College Park,Rose [22] Filed: Sept. 21, 1971 [57] ABSTRACT 211 Appl 132 A digitaldirection finder system for radio waves in which the individual bits ofthe code indicating direction are obtained by comparing and successivelypar- [52] U.S. Cl. 343/113 R, 343/100 R tially resolving errors in thedigitized phase angle i [51] Int. Cl. .1 G015 5/02 placement indicatedby plural pairs of antennas having [58] Fleld of Search 343/113 Rprogressively different Spacing between the respective pairs. Thespacing between the closest pair of anten- [56] References Cted nas ismore than one-half of the wavelength so that UNITED STATES PATENTSlarger and higher gain antennas may be employed. 3,125,756 3/1964Kaufman et a1 343/113 R The System y be Operated Over a Wide range of3,213,453 10/1965 Morrison et a1. 343/113 R quencies where the highestfrequency may be many 3,249,943 5/1966 Kaufman 343/113 R times greaterthan the lowest frequency. 3,406,397 10/1968 Easton et a1. 343/113 R 11Claims, 11 Drawing Figures e3/ e= ea4.5 /l.s= a5/11:

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16 CELLS l6 CELLS 16 0151.15 4 CODE RESOLVER RESOLVEFl 20 A 16 CELLS B48 CELLS 48 CELLS RESOLVER 240 CELLS $400 CELLS PATENTEL BEE 3W4 /5b 25c 2/ 5d 210 ED M MEMORY SHEET 20F 3 LEADS IO LEADS SELECTION FROM LOGCRESOLVERS 1O BIT ADDRESS M2 BIT Q, 17/ 70 a w INV. 59 68 V y I;

T lOBlT a ADDRESS ROM 73 AN? 74 km ROM ROM

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PATENIELBEE 3W 3.8529754 sum 3 F F B H I6 l4 :2 I ll INTEGRATED (W3 0Fig) MEMORY FIG.'+

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QUANTIZER /4 r5 QUANTIZER MULTlPLIER MULTIPLIER 83 DlSPLACE 82 1 9 GBITJSUBTRACT WWJ 2 BIT OUTPUT MATRIX BINARY BEAM SYSTEM BACKGROUND ANDSTATEMENT OF THE INVENTION Interferometers for determining the spatialdirection of incoming radio waves referenced to an array of spacedantennas are known wherein the direction of the wave is determined bymeasuring the phase differences of signals received by the antennas.Where a pair of the antennas are spaced apart by only one half thewavelength or less at the frequency involved, the measured phasedifference of the received signal is never greater than 360and thedirection of the incoming wave can be coarsely determined inanunambiguous fashion. However when the antenna pairs are more widelyspaced apart such as at once, twice, three times, or even greatermultiples of the wavelength, than the measured phase delay of thereceived signal corresponding varies over multiples of the 360andtherefore creates many ambiguities as to the spatial angle.

Where the interferometer is to be employed over a wide range ofdifferent frequencies, such as over a percentage band-width of six toone, than the spacing be tween antenna elements at the lowest frequency(Iongest wavelength) may be only one-half wavelength, yet the samephysical spacing at the highest frequency is a multiple of three timesthe higher wavelength. Consequently as the frequency range, orpercentage bandwidth, of the system is increased, practicalconsiderations dictate that such a system be capable of determining thedirection without ambiguity despite measured phase differences occuringover multiples of 360.

SUMMARY OF THE INVENTION AND ADVANTAGES According to the presentinvention there is provided a system for digital interferometricdirection finding or location of radio signals that determines thespatial di rection of the wave by comparing the different time phasedisplacements produced by different pairs of antenna elements that arespaced at different distances apart. To obtain the advantages of highergain and efficiency as well as lower costs, the minimum spacing betweenany pair is more than one-half of the wavelength at the maximumfrequency involved.

Such wide apart spacing of the antenna elements per mits the use ofhigher gain and higher efficiency antennas, even at the higher microwavebandwidths. However, correspondingly, the wide spacing provides numerousambiguities, or like signals being produced by each pair of antennas fordifferent spatial angle of the incoming wave.

Due to the different spacing between each pair of antennas over theothers, an incoming wave produces a different phase shift in each pairof antennas, and the ambiguities produced by each pair accordingly occurat different spatial angles of the incoming wave. This characteristic ofthe system is employed to enable the successive resolution of theambiguities and the obtaining of the desired directional informationwith the desired degree of resolution.

To provide a digitally operating system, the time phase displacedsignals obtained from the pairs of differently spaced antennas arequantized and the quantized phase signals are combined with each otherto define the spatial direction of the incoming wave in a fullyunambiguous digital code as desired.

RELATED APPLICATIONS OF THE SAME ASSIGNEE In an earlier application ofthe same Assignee Ser. No. 501,231 now US. Pat. No. 3,631,496 of CharlesFink et al, there is provided an interferometric direction findingsystem employing different digital means, including amplitudecomparison, to determine the spatial angle of an incoming wave.

A further application of the same inventors now US. Pat. No. 3,617,900employs different digital comparison means for determing the frequencyof. such an in coming wave.

' DESCRIPTION OF THE DRAWINGS FIG. 1 is an electrical schematic blockdiagram illustrating one preferred embodiment of the invention,

F IG. 2 is a diagrammatic view illustrating the spatial relationship ofan incoming wave referenced to the array of antennas,

FIGS. 3a to 3c are wave form diagrams illustrating the change in phaseangle between pairs of antennas plotted against change in spatialdirection of the radio wave,

FIG. 4 is a graphic plot illustrating the change in phase of one pair ofantennas referenced to the change in phase of a second pair of antennas,

FIG. 5 is a block diagram illustrating one embodiment of resolver 20 or21 of FIG. ll,

FIG. 6 is a similar block diagram showing one embodiment of resolver 22of FIG. 1,

FIG. 7 is a block diagram showing details of the selection logic circuitof FIG. 8,

FIG. 8 is a block diagram showing the interconnection of plural smallcapacity ROM, and

FIG-9 is a block diagram showing an alternate computer for resolving thedigital codes.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. I,there is shown a preferred digital system for determining the spatialdirection of an incoming wave in azimuth within an accuracy of less than0. 1 RMS on boresight at the maximum frequency; and operation over morethan a 6 to 1 frequency band.

As shown, the system includes an array of antenna elements 10, ll, 12,and 13, that are employed in five pairs, and with each pair being spaceda different distance apart. A first pair of antenna elements 10 and 11are spaced apart by three wave lengths at the frequency involved;elements 1 land 12 forming the next pair are spaced apart at 4 /2wavelengths; elements 10 and 12, forming the next succeeding pair, arespaced at 7 /2 wavelengths apart; elements 12 and 13 are spaced at 5wavelengths apart; and finally elements 10 and 13 forming the remainingpair are spaced at 12% wavelengths apart. It will be appreciated that atotal of IO descrete antenna elements may be employed to provide thefive pairs of antennas desired, instead of using different combinationsof the four antenna elements as shown. It will also be understood thatother arrangements using different numbers of antennas may be employed.

The time phase relationship of received signals is determined from eachantenna pair by employing a phase comparator and quantizer for each pairof antennas. Thus the phase difference between the antenna elements and11, spaced apart by three wavelengths, is determined by phase comparatorand quantizer l4, and this unit 14 produces a four digit output codeproportional to the phase difference. Similarly the phase differencebetween antenna elements 11 and 12 is compared and quantized bycomparator 15; that between antennas 10 and 12 by comparator 16; thatbetween antennas 12 and 13 by comparator 17; and finally that betweenantennas 10 and 13 by comparator 18.

In this manner there is derived five different digital codes, with eachcode representing the different time phase produced by a pair ofdifferently spaced antennas in response to an incoming wave directed atan space angle to the linear array of these antennas.

The reason for each of the phase signals being different and alsochanging at a different rate from the others is best illustrated in FIG.2 showing the spatial relationship of an incoming radio wave 19 receivedby the four spaced apart antenna elements. As shown where the incomingwave 19 is received at a spatial angle 0, referenced to the boresight ofthe antenna array, the wavefront is first received by antenna 10, andafter a given time phase delay by antenna 11, then later by antenna 12,and finally by antenna 13. Thus for each different space direction ofthe incoming wave, all four time phase delay signals are different. Forany given spatial angle it can be shown that the time phase delay 15between any pair of antennas is directly proportional to the spacing ordistance between that pair of antennas, (measured in wavelengths at thefrequency involved). It is also known that the time phase delayisproportional to the sine of the spatial angle referenced to boresight.Stated mathematically:

(b 360 D-rX sin 0 where is the time phase delay D is the distancebetween antennas in wavelengths r is the wavelength at the frequency,and

0 is the spatial angle of the incoming wave.

Thus for antenna elements 10 and 11 that are spaced apart by threewavelengths, the time phase difference (or delay) is:

From this formula it is seen that as the space angle of the incomingwave varies from 0 to 90, the phase difference between antenna elements10 and 11 passes through three repetitive cycles of 360 time phasedelay; the phase angle between antenna elements ll and 12 goes throughfour and a half repetitive cycles of 360 delay, that between antennas l0and 12 passes through seven and a half complete 360 phase delays; thatbetween antennas l2 and 13 passes through five complete 360 phasecycles, and finally that between antennas l0 and 13 passes throughtwelve and a half complete 360 time phase cycles.

Returning to FIG. 1, it is therefore seen that each of the phasecomparator quantizers 14, 15, l6, l7, and 18 produces a different phaseangle code than the others for each different direction or spatial angleof the received input radio wave 19, and that each of these codes beingproduced repeats itself a number of times as the direction of theincoming wave changes, in proportion to the spacing between that pair ofantennas.

According to the present invention it is preferred that the minimumspacing between the closest pair of antennas 10 and 11 be a more thanone half of the wavelength and preferably be several wavelengths apart(i.e., 1, L5, 2.3, 3). This wide apart spacing is desired so thatphysically larger antenna elements can be employed having greater gainand higher efficiencies. It will be appreciated that as the system isemployed at higher and higher microwave frequencies, the physicaldimensions of a one wavelength spacing become smaller and smaller. Thusto provide a fixed physical spacing between the closest pair of antennaelements, requires that the spacing expressed in wavelengths increasesin directproportion to the frequency.

The functioning of the remainder of the system of the present inventionis to digitally process the five different digital phase codes beingproduced so as to accurately define the spatial direction of theincoming wave in an unambiguous fashion and with the desired degree ofprecision, e.g. within an angular direction of a tenth degree or less.

To successively determine the direction of the wave and resolveambiguities, the different digital codes from the comparator-quantizer14 and from the comparator-quantizer 15 are applied to a digitalresolver 20 which performs the function of eliminating certain ones ofthe ambiguities and producing a modified digital code defining thedirection within the resolution of the 4 /2 wavelength antenna pair 11and 12, yet with fewer ambiguities.

Similarly, the digital codes from quantizers 16 and 17 are applied to adigital resolver 21 serving a similar function of reducing theambiguities inherent in the seven and one half wavelength spacedantennas yet maintaining the resolution of this pair. This additionalresolver 21 produces a modification in the code produced by quantizer16.

In the next succeeding level of the preferred system, the modified codesfrom resolvers 20 and 21 are still further resolved by application toanadditional resolver 22, and in this resolution all of the remaindingambiguities are eliminated and the incoming wave is defined in itsspatial direction with the accuracy or resolution provided by the sevenand one half wavelength spacing of antennas 10 and 12. In short, thespatial angle of the wave over a field of view is defined within anangular direction of 0. 14 degrees RMS on the antenna boresight.

For improving the accuracy even further to spatially define the incomingwave within a 0.08 direction in space, a still additional resolver 23 ispreferably employed. Resolver 23 is energized by the digital code signalfrom quantizer 18 and from the signal obtained from higher levelresolver 22 to provide a still further modification of the resultingcode, defining the wave within the inherant accuracy of the twelve andone half wavelengths antennas 10 and 13 or to within a spatial sectordirection of less than 0.l RMS on boresight.

For an understanding of the manner of resolving the ambiguities andimproving accuracy by successive digital resolution, reference is madeto FIG. 3 illustrating the variation of the phase output of the phasecomparators 14, 15, 16, 17, and 18 as the space angle 6 of the incomingradio wave 19 changes from boresight (see FIG. 2) over the range of 0 to90. To provide a simplified illustration using straight line variations,the phase outputs of the quantizers are plotted against the sine of thespace 0 rather than against the space angle 0 itself.

As shown in the upper plot, of FIG. 30, (solid line plot) as the spaceangle varies from sine 0 to sine 90, or from 0 to I, the magnitude ofphase difference output from the 4% wavelength spaced apart antennas 12and 11 (quantizer 15), varies from 0 to 360 in a repetitive manner,repeating itself four and one times as follows:

Since for practical measurement purposes, a system cannot distinquishbetween an output time phase angle of and one of 370 the output ofquantizer repeats itself or produces a total of 4 /2 ambiguities in each90 variation of the space angle 0. In other words, the 4% wavelengthquantizer 15 produces the same phase difference output code in each of 4/2 different space sectors or quadrants. For example, as shown in FIG.3a, the same 360 time phase angle (b is produced by quantizer 15 atthose four different space angles ((11) where sine 6 4/9, and sin 0= 2/3and sin 0 =-2/9, and sin 6 8/9. In a similar manner, the 3 wavelengthsspaced quantizer 14 (dotted line plot) repeats its code three times asthe space angle 0 is varied from 0 to 90, and thus produces threeambiguities or provides three different space quadrants where the phaseangle varies from 0 to 360.

However by resolving the codes of quantizers l4 and 15 against eachother, there is provided a pair of digital codes defining each spaceangle that changes at a different rate than either code, therebyenabling the generating or derivation of a new digital code having fewerambiguities. Returning to FIG. 3a, it is seen that at the discussedangle where sin 0 equals 2/9, quantizer 15 produces a phase outputrepresenting of 360 whereas the quantizer 14 produces a phase output ofabout 240. In the different space quadrant where sin 0 equals 4/9, thequantizer 15 again produces a phase output of 360 thereby producing anambiguity. However at this same latter space angle, the quantizer 14(three wavelength spaced antennas) produces a different output of about120. Therefore employing the outputs of both in a resolver, there is nolonger an ambiguity between these two space quadrants since by using thetwo outputs one can distinguish between an incoming wave at the spaceangle where sin 0 2/9 and the different space quadrant where the sin 64/9.

In a similar manner it is seen from FIG. 3a that in the different spacesector where sin 0 equals 2/3, the quantizer 14 again produces anambiguous output signal representing a phase angle of 360 whereas thesecond quantizer 15 again corrects this ambiguity by producing an outputrepresenting an angle of 360. Thus by resolving the output of quantizer14 against that of quantizer 15, all ambiguities over the range of spaceangles from sin 0 to sin 6 equals 2/3 (space angles from 0 to about 43)are resolved. After passing this later space angle, however, both codescommence to repeat as shown in FIG. 3a, and further resolution isnecessary. For example, at the space angle when sin 0 equals 8/9, (about62) the output code of quantizer 15 represents a phase lag of 360 andthat of quantizer 14 represents a lag of 240. These are the same pair ofoutputs as occur at the space angle when 6 equals 2/9 (at a space angleof about 13) as illustrated in FIG. 3a.

To resolve this latter ambiguity extending from about a space angle of43 where sin 0 equals 2/3 to a space angle of the additional resolver 21is employed to compare the quantized signals produced by quantizers 16and 17. As previously discussed, the output of quan tizer 16 repeatsthrough a complete phase delay of 360 for seven and one half times asthe incoming space angle 6 of the wave changes from 0 to 90 spatialdegrees since the antenna elements 10 and 12 are spaced apart by sevenand one half wavelengths. In a similar manner the output of quantizer 17repeats or cycles through a complete phase delay of 360 a total of 5times during the same changes in the incoming space angle. Thesevariations are plotted in FIG. 3b.

Again in resolving the output of quantizer 16 against that of quantizer17 in digital resolver 21, there is compared an additional pair ofdigital codes for each spatial angle 0 of the incoming wave as shown inFIG. 3b, and by.still further combining or resolving this additionalpair of codes with the first pair of codes in a still further resolver22, atota] of four digital codes are available and employed in theresolvers for defining each angle of the incoming wave without anyambiguities over the complete range of space angles of 0to 90 spacedegrees.

Referring to FIG. 3b for a graphic illustration showing the solution ofall ambiguities, it will be recalled that after the first resolution byresolver 20, the space angle is defined without ambiguity over a rangeof from 0 to about 43 from boresight. From about this 43 angle in spaceto 90 in space, the pair of outputs provided by quantizers 14 and 15begin to repeat. However, as shown in FIG. 3b, the pair of codesproduced by quantizer 16 (7 /2 wavelengths spaced antenna) and quantizerl7 (5 wavelength spaced antennas) cyclicly repeat their phase changes atdifferent rates and in differing phase relationships than those ofquantizers l4 and 15 (in FIG. 3a). Consequently by comparing the codesof quantizers 16 and 17 using resolver 21, and in turn, comparing orfurther resolving the resulting output against that obtained employingresolver 20, the spatial angle of the incoming wave can be defined byfour different outputs in a completely unambigous manner.

For example, it will be recalled from FIG. 3a, that at the space angle 0where sine 0 equals 2/9, the two outputs obtained from quantizers 14 and15 are the same two as those obtained at the space angle where 6 equals8/9. However, observing FIG. 3b, it is seen that at this space anglewhere sine 0 equals, 2/9, the quantizer 17 (five wavelengths) producesan output representing a phase of about 36 and quantizer 16 (7 /2)produces an output phase of about 280. At the other space angle wherethe sine equals 8/9, the quantizer 17 produces a different phase outputequaling about 180 and quantizer 16 also produces a different phaseoutput equaling about 280.

In a similar manner, it is seen by comparing the waveforms of FIGS. 3awith FIG. 3b that all of the ambiguities are resolved and that at eachdifferent space angle 0 of the incoming wave, a different series of fouroutputs from quantizers 14, 15, 16, and 17 exist from which anunambigous digital output code can be derived from the resolvers.

Thus by processing the four quantized output phase signals from phasedelay quantizers 14, 15, 16, and 17, and resolving these four outputsagainst each other employing resolvers 20, 21, and 22 there is obtaineda modified digital output code defining the spatial direction of theincoming wave referenced to the antenna array without any ambiguitywhatsoever over the entire range of to 90 in space. However the accuracyor resolution of this resulting digital code is only as great as thechange in phase angle experienced by the most widely spaced apart pairof antennas, which as thus far described are the antenna pair 10, 12,that are spaced apart by seven and one half wavelengths as shown in FIG.3b. The region from 90 to 0 has not been portrayed but it also iscompletely unambiguous and has no point of ambiguity with the 0 to 90region.

Assuming that each phase comparator quantizer provides a four bit binarycode output to represent all angles within a complete 360+ phasevariation, then it can represent changes in phase over its completerange by a total of 16 different output codes. A resolution provided bya seven and one half wavelength spacing of antennas provides a total ofi6 times 7% or 120 different code combinations to represent thevariation of the space angle from 0 to 90 in space.

To further improve this resolution. an additional phase output code maybe obtained by comparing the phase relationship between the furthestapart antennas l0 and 13, that as shown, are spaced apart by 12%wavelengths. This additional spacing produces tweleve and one halfcomplete repeats of 360 output phase change as the space angle of wave19 varies from 0 to 90 in space, as illustrated in FIG. 30. Since each360 phase change produces 16 different four bit digital codes inquantizer 18, the incoming beam 19 is now defined by a combined digitalcode of 16 times 12% or by two hundred different code combinations. Thequantized output code produced by quantizer 18 is finally compared withthe resulting code produced by resolver 22 in the final resolver 23 tothereby improve its resolution accordingly.

It is usually desired to determine the spatial direction of the incomingradio wave over the space angle on both sides of boresight or from 0 to90 in space as well as from 0 to 90 in space as previously discussed.The 0 direction merely represents the fact that the incoming radio waveis arriving from the opposite side of boresight than as previouslydiscussed, and this is generally shown in FIG. 2 by the dotted linevector numbered 24. In the case, the antenna phase relationships arereversed with the wavefront reaching antenna 13 first, then after adelay being received by antenna 12, thence by antenna 11, and finally byantenna unit 10. The output phase angle plots produce the same waveformsas are shown in FIG. 3a, FIG. 3b, and FIG. 30 but transposed on theother side of the ordinate from the 0 space angle. It is seen bycomparing the waveform of FIG. 30 with those of FIGS. 3a and FIGS. 3bthat since the rate of phase change with space angle becomesprogressively greater in proportion to the extent of spacing betweenantennas, the phase change is most rapid for the antenna elements and 13that are most widely spaced apart by twelve and one half wavelengths andtherefore the four bit output digital code produced by quantizer 18changes a total of 200 times over the range space angles from 0 to 90 inspace and an additional 200 times over the range of space angles from -0to in space. It therefore changes a total of 400 times over the entirerange of 90 in space to +90 in space. This provides an angularresolution of 0.083 RMS on boresight.

In FIG. 4, the digitally represented phase output from the pair ofantennas 11 and 12 (spaced apart by four and one half wavelengths) isplotted against the digital code output produced from the pair ofantennas spaced apart by three wavelengths (antennas 10 and 11 Thesedigital outputs are those obtained from comparator-quantizers 15 and 14respectively. As previously noted each of the digital quantizers producea four bit code and each therefore produces sixteen cells or descretevariations of the four digit code to represent a 360 output phase.

As shown, this digital plot illustrates that over the range of codechanges or digital cells represented, there is provided a uniquecombination of two codes for each different space angle of the incomingwave. Thus, for example, where the digital code produced by quantizer 15(4 /2) is located in the first cell, which might occur at any one offive different space sectors of the incoming wave, this ambiguity isresolved by referring to the output of the quantizer 14 which may beconcurrently generating a code of cells 1, 6, or 11. If the quantizer 14is producing the code of cell 1 than the incoming wave is defined alongthe line 4a in FIG. 4 which is a unique plot covering the range of sin 0equals 0 to sin 0 equals 2/9 (the space sector from 0 to about 13).Similarly if the output code of quantizer l4 falls in the second cellwhile concurrently that of quantizer l5 falls in the eleventh cell, thenthe incoming wave is uniquely defined along the line 4c of FIG. 4covering the range of sin 6 equals 2/9 to sin 6 equals 4/9, or where thespace angle is in the sector from about 13 to about 26/a.

In this plot it can be' shown that the line 4a uniquely covers the spaceangles of 0 to about 13, lines 4b and 40 together cover the space sectorof from about 13 to 26/2, and line 4d uniquely covers the space sectorfrom about 26/2 to about 42. In this overall range of from 0 to 42 ofthe space angle, the wave is uniquely defined by the two output codes ofquantizers l4 and 15. However from about 42 to 90, these codes begin torepeat themselves creating ambiguities as discussed above in connectionwith FIG. 3. As previously discussed these ambiguities are resolved byemploying the additional output phase codes produced by quantizers l6and 17 that are resolved against each other, and the result thereof isthen further compared or resolved against the code derived fromquantizers l4 and 15.

A two dimensional digital chart or plot similar to FIG. 4 can beprovided for each of the other resolvers 20, 21, 22, and 23 showing thederivation of unique codes for each of the 7% and 5 wavelengths, and theothers.

In FIG. 5 is shown a block diagram illustrating one embodiment ofresolver 20 in FIG. 1, or resolver 21 in FIG. 1. As will be recalled,the function of resolver 20 is to receive a four digit code fromquantizer l5 (3 wavelength spacing) and a four digit code from quantizerl5 (4% wavelength) and produce a six digit output code that resolves anyambiguities over a range of space angles extending from sin 0 of 0 tosin 0 of 2/3, or over a space angle sector of from 0 to 43 (on eitherside of boresight.)

In this preferred embodiment, a conventional on-theshelf, integratedcircuit Read-only-memory 50 is employed, of the type having a total of256 storage positions or locations. This memory 50 is preprogrammed toproduce a preselected two digit code over output lines 51 and 52 wheninterrogated at each position by the eight digit code input address fromquantizers 14 and 15 over input lines 14a to 140. and 15a to 15d. Aswill be recalled the four digit output code on lines 15a to 15d fromquantizer 15 (4% wavelength) defines the spatial direction or angle ofthe radio wave as being within any one of four and one half spatialsectors (on opposite sides of boresight) as shown in FIG. 3a. Thereforethe two digit code preprogrammed within ROM 50, in combination with thefour digit code obtained from quantizer 15, provides a resulting sixdigit code that defines the space angle 0 of the radio wave within thespace sector referred to above without ambiguity.

A similar read-only-memory is employed to provide resolver 21 thatresponds to quantizers l6 and 17 in FIG. 1.

As shown by FIG. 6, the next level resolver 22 on the other hand,requires many more storage positions since it responds to a six digitcode from resolver 20 as well as a further six digit code from resolver21. The 12 digit input code, or address, therefore requires a memorycapacity of about 4000 positions and produces as an output a four digitcode over lines 55, 56, 57, and 58 which is combined with the outputcode from 210 to 21f obtained from quantizer 17 in FIG. 1 to define thespace angle of the wave over its complete sector of 0 to 90 (both sidesof boresight).

Since on the shelf, integrated, read-only-memories of this largercapacity are not presently available, a series of smaller capacitymemory units are combined as shown in FIGS. 7 and 8 to provide thislarger capacity storage device.

As shown in FIG. 8, there is provided a group of five larger capacitycomposite read only memories, 60, 61, 62, 63 and 64. Each of thesememories has a combined storage capacity of at least 4096 bits. Of thetwelve bit output code from resolvers 2t and 21, ten of the bits areemployed as an address applied to all of the composite memory units 60to 64, and the remaining two of the bits of the code are applied to aselection logic circuit 64 to select that portion of each of thecomposite memory units to be so addressed. As noted above, all of theseintegrated memory units are fixedly preprogrammed to respond to theaddressing code and produce an eight bit output code over lines 55 to 59and 210 to 21f, as shown in FIG. 6. This resulting code fully resolvesthe space angle direction 0 of the incoming radio wave within theaccuracy of the quantizer 16 (7% wavelength antennas FIG. 3b).

FIG. 7 illustrates the functioning of the selection logic means 65 ofFIG. 8 and the manner of combining plural integrated circuit read onlymemory units to produce each of the composite units such as 60 of FIG.8.

'As shown, the composite unit 60 is comprised of four individual memoryunits 60a, 60b, 60c, and 60d having their outputs connected in parallelto line 59 and their inputs connected in common to be addressed by a tenbit code. Each of these memory units such as 60a is presently availableas a storage unit having a total of 10 nations of l and 0 bits on lines70 and 71 activates or gates a different one of the memory units 6011 to60d, thereby to select that unit. It is believed now evident, that eachof the other composite read-only-memories 61 to 6 1 of FIG. 8, functionsin this same manner as described. It is believed also now evident thatthe circuitry of FIGS. 7 and FIG. 8 is required only because of theunavailability of a single integrated circuit readonly memory unithaving a large enough storage capacity. When such larger capacityintegrated units becomes available, they may be employed as a completedunit to eleminate the circuitry of FIGS. 7 and 8.

The circuitry for the final resolver 23 of FIG. 1, is similar to thatfor resolver 22 in FIG. 6, comprising a device responding to-an inputcode of twelve bits for 25 interrogating the memory (eight bits from theoutput of resolver 22 and four bits from quantizer 18). The output codebeing produced is a nine bit code, with five bits being obtained fromthe interrogated memory unit and the remaining four bits being obtainedfrom quantizer 18.

It will be recalled that the spatial direction of the incoming wave isobtained without ambiguity over the full angular sector of 0 to plus orminus 90 from boresight, from the output code of resolver 22. Thepurpose of adding additional resolver 23 and quantizer 18 is to obtaingreater resolution 'of the spatial direction of the incoming wave.

An alternative manner of resolving the digital codes against each otheris by a computational technique as illustrated in FIG. 9. As is shown inFIG. 4, when the output of quantizer 14 (3 wavelength) is plottedagainst the output of quantizer 15 (4 /2 wavelength) there is produced aseries of displaced straight lines 4a, 4b, 4c, and 41d, all of whichhave the same slope and differ only by the position at which theyintercept the horizontal axis of the graph. For the plot of a 4 /2wavelength spacing of antennas against 3 wavelengths, the ratio or slopeis 3/2, or 3 to 2. Therefore one can calculate the intercept for each ofthese displaced lines 4a to 4d, and therefore determine by computationthe space sector of the incoming wave.

Mathematically this can be expressed as:

Q slope X (Q intercept) where 0 is the phase output of quantizer 15;

Q; is the phase output of quantizer 14; and

the slope is 3/2 for quantizers 14 and 15.

Transposing this equation, one can find the intercept" for each line as:

intercept Q 2/3. 0.,

3. intercept 3.Q 2.0.,

FIG. 9 illustrates a special purpose digital calculator circuitry forperforming this calculation. As shown, the four digit or four bit outputfrom quantizer l4 (3 wavelengths) is applied to a multiplier to producea six bit output code corresponding to its multiplication by a factor ofthree. The four bit code of quantizer 15 (4V2 wavelengths) is multipliedby a factor of two in multiplier 81 to produce a five bit output code.To this five bit code is added a fixed number displacement unit toderive a six bit code output as shown. This displacement is forconvenience in processing.

The resulting six bit numbers are then subtracted from one another in asubtraction circuit 83, thereby yielding a seven bit output. This sevenbit output is a unique number representing the intercept and thereforedefining that one of the four lines 4a, 4b, 4c, and 4d of FIG. 4 thatthe incoming radio wave is producing. It will be recalled that where theincoming radio wave produces phase coordinates from quantizers 14 and 15falling along line 4a of FIG. 4, then the space sector of the incomingwave is in the sector of in space to 13 in space. Similarly, a waveproducing coordinates along line 40 falls in the range of 13 in space to26 /2, and so forth. Thus the digital code generated by the calculatorof FIG. 9 uniquely defines the angular space sector of the incoming waveand this number is processed through a matrix 84 to generate a resultingtwo bit code defining this angular sector in the same manner as does thetwo bit output code on lines 51 and 52 of FIG. produced by theread-only-memory embodiment. This two bit code produced by computationis. combined with the four bit code from quantizer 15, as previouslydiscussed, to define the direction of the incoming wave.

In a similar manner as described in FIG. 9, each of the other resolvers22, and 23 may be performed by calculation, rather than by the use ofread-onlymemories to produce the unique coding required to distinguishbetween ambiguities and to define the space direction of the incomingwave.

It will be apparent to those skilled in this art that general purposecomputer may also be used for calculating the resolved terms, ratherthan the special purpose fixed programmed units of FIG. 9.

Many other changes and variations may be made without departing from thespirit and scope of this invention and accordingly this invention shouldbe considered as being limited only by the following claims.

I claim: 1. A digital direction finding system comprising: an arrayincluding a first pair of antennas spaced apart at a distance that is agreater multiple than /a of the wavelength of the signal to be detected,

said array including antenna means providing a second pair of antennashaving a spacing greater than the first pair,

first and second phase comparator means for determining the time phasedisplacement of signals from each pair and providing first and secondsignals proportional to such phase displacements,

and a resolver energized by said first and second signals for producinga unique digital code defining the direction of an incoming wave withfewer ambiguities than the signal from either phase comparator.

2. In the system of claim 1, additional antenna elements in said arrayproviding at least one additional pair of elements having a differentspacing than either of said two pairs.

an additional phase comparator for determining the phase displacement ofthe signals from additional pair,

and an additional resolver energized by signals from said resolver andsaid additional phase comparator for further resolving ambiguities insaid code.

3. In the system of claim 1, at least four antenna elements providingfive pairs of antennas having different spacing, with the most closelyspaced pair being spaced apart at a multiple of the wavelength,

a phase comparator for each such pair providing a signal proportional tothe phase displacement for each pair,

a first and second resolver each energized by a different pair of saidphase comparators and providing first and second resolved digital codeoutputs,

a third resolver energized by said first and second resolved code outputto product a next order resolved code output signal,

and a fourth resolver energized by a signal obtained from the furthestapart pair of antenna elements and additionally energized by the outputsignal obtained from the third resolver to further eliminate ambiguitiesin the digital code.

4. In the direction finding system of claim 1, quantizer meansresponsive to each comparator for producing quantized signalsproportional to the phase displacements, and said resolver energized bysaid quantized signals.

5. In the direction finding system of claim 1, said resolver including apreprogrammed read-onIy-memory.

6. In the direction finding system of claim 1, said resolver includingmeans for computing the unique intercept of a linear plot relating thefirst and second signals.

7. In the system of claim 1, said first pair of antennas spaced apart bythree wavelengths, said second pair spaced apart by 4 /2 wavelengths,and three additional pairs of antennas spaced apart by 7 /2 wavelengths,five wavelengths, and 12 /2 wavelengths.

8. In the system of claim 1, quantizing means responsive to each pair ofantennas for producing a digital code representing the phasedisplacement, and said resolver energized by said digital codes forderiving a modified code having fewer ambiguities.

9. In the system of claim 3, each said phase comparator includingquantizing means for providing a digitized signal proportional to thephase displacement from each pair, and said resolvers each energized bysaid quantized signals and producing a quantized code output.

10. In the system of claim I, the relative spacing of said first andsecond pairs of antennas producing ambiguities in phase that occur atdifferent space angle directions of the incoming wave.

11. A digital direction finding system comprising a plurality of pairsof differently spaced apart antennas,

the minimum spacing of anyone of said pairs being greater than one halfwavelength at the frequency of an incoming wave,

the spacing of all others of said pairs being different from eachother,

quantizing phase comparator means producing plural digital codes eachrepresenting the phase displacement between each different pair ofantennas,

plural resolver means for successively comparing different ones of saiddigital codes to derive modified codes having fewer ambiguities than thecompared codes,

the relative spacing of said different pairs of antennas providingambiguities at different space angles of

1. A digital direction finding system comprising: an array including afirst pair of antennas spaced apart at a distance that is a greatermultiple than 1/2 of the wavelength of the signal to be detected, saidarray including antenna means providing a second pair of antennas havinga spacing greater than the first pair, first and second phase comparatormeans for determining the time phase displacement of signals from eachpair and providing first and second signals proportional to such phasedisplacements, and a resolver energized by said first and second signalsfor producing a unique digital code defining the direction of anincoming wave with fewer ambiguities than the signal from either phasecomparator.
 2. In the system of claim 1, additional antenna elements insaid array providing at least one additional pair of elements having adifferent spacing than either of said two pairs, an additional phasecomparator for determining the phase displacement of the signals fromadditional pair, and an additional resolver energized by signals fromsaid resolver and said additional phase comparator for further resolvingambiguities in said code.
 3. In the system of claim 1, at least fourantenna elements providing five pairs of antennas having differentspacing, with the most closely spaced pair being spaced apart at amultiple of the wavelength, a phase comparator for each such pairproviding a signal proportional to the phase displacement for each pair,a first and second resolver each energized by a different pair of saidphase comparators and providing first and second resolved digital codeoutputs, a third resolver energized by said first and second resolvedcode output to product a next order resolved code output signal, and afourth resolver energized by a signal obtained from the furthest apartpair of antenna elements and additionally energized by the output signalobtained from the third resolver to further eliminate ambiguities in thedigital code.
 4. In the direction finding system of claim 1, quantizermeans responsive to each comparator for producing quantized signalsproportional to the phase displacements, and said resolver energized bysaid quantized signals.
 5. In the direction finding system of claim 1,said resolver including a preprogrammed read-only-memory.
 6. In thedirection finding system of claim 1, said resolver including means forcomputing the unique intercept of a linear plot relating the first andsecond signals.
 7. In the system of claim 1, said first pair of antennasspaced apart by three wavelengths, said second pair spaced apart by 41/2 wavelengths, and three additional pairs of antennas spaced apart by7 1/2 wavelengths, five wavelengths, and 12 1/2 wavelengths.
 8. In thesystem of claim 1, quantizing means responsive to each pair of antennasfor producing a digital code representing the phase displacement, andsaid resolver energized by said digital codes for deriving a modifiedcode having fewer ambiguities.
 9. In the system of claim 3, each saidphase comparator including quantizing means for providing a digitizedsignal proportional to the phase displacement from each pair, and saidresolvers each energized by said quantized signals and producing aquantized code output.
 10. In the system of claim 1, the relativespacing of said first and second pairs of antennas producing ambiguitiesin phase that occur at different space angle directions of the incomingwave.
 11. A digital direction finding system comprising a plurality ofpairs of differently spaced apart antennas, the minimum spacing ofanyone of said pairs being greater than one half wavelength at thefrequency of an incoming wave, the spacing of all others of said pairsbeing different from eachother, quantizing phase comparator meansproducing plural digital codes each representing the phase displacementbetween each different pair of antennas, plural resolver means forsuccessively comparing different ones of said digital codes to derivemodified codes having fewer ambiguities than the compared codes, therelative spacing of said different pairs of antennas providingambiguities at different space angles of the incoming wave.